Technical Field
The present disclosure relates to an electromagnetic flowmeter, and more particularly, to detection of wear of a lining which covers an inner side of a measurement tube.
Related Art
An electromagnetic flowmeter for measuring a flow rate of conductive fluid by using electromagnetic induction is widely used in an industrial application or the like since it is robust and has high precision. The electromagnetic flowmeter is configured such that conductive fluid to be measured is supplied into a measurement tube to which a magnetic field is orthogonally applied and a generated electromotive force is measured by using a pair of electrodes. Since this electromotive force is proportional to a flow velocity of the fluid, a volumetric flow rate of the fluid can be obtained based on the measured electromotive force.
FIG. 7 is a sectional view of an electrode part of a related-art electromagnetic flowmeter 300. As shown in FIG. 7, an inner side of a measurement tube 310 is covered with a lining 320 and an exciting coil 340 is provided on the outside of the measurement tube 310.
The lining 320 has a role of securing wear resistance and corrosion resistance of the measurement tube 310 to the fluid in addition to securing insulation property of the measurement tube 310 to an electromotive force. Material of the lining may be selected according to the fluid to be measured and may be, for example, a fluorine resin (PFA, PTFE), a polyurethane rubber or a soft natural rubber and the like.
In the example shown in FIG. 7, a pair of electrodes 330 are machined to have a length such that a contact side surface with the fluid is flush with the surface of the lining 320. The pair of electrodes are assembled to the measurement tube 310 by being inserted from the outside thereof. As material of the electrodes 330, a metal material according to the fluid to be measured may be used.
In the measurement of slurry fluid containing solids referred to as slurry, such as gravel, sand, mud, metal pieces and plastics, the lining 320 is gradually worn over time as the slurry is brought into contact with the lining 320. When the lining 320 is worn, a channel diameter in the measurement tube 310 is changed, and thus, a measurement error referred to as a span error would occur.
In relation to the wear of the lining 320, JP-A-2006-250692 discloses a configuration where as shown in FIG. 8, a wear detection electrode 351 is embedded into the lining 320 from the measurement tube 310 and a voltage is applied between the wear detection electrode 351 and a reference potential (earth) of the measurement tube 310.
In the state where the lining 320 has no wear, the wear detection electrode 351 is not in contact with the fluid to be measured, and thus, a current does not flow between the wear detection electrode 351 and the earth. However, when the wear of the lining 320 advances and the wear detection electrode 351 is exposed, a current Ia flows between the wear detection electrode 351 and the earth through the fluid. By monitoring the current Ia, the wear of the lining 320 can be detected. Further, by providing a plurality of wear detection electrodes 351 at different heights, the degree of wear of the lining 320 can be detected.
According to the above configuration, the wear of the lining 320 can be detected. However, the wear detection electrode 351 needs to be embedded into the lining 320. Further, a circuit for monitoring the current Ia needs to be separately provided, and therefore, a configuration of an electromagnetic flowmeter becomes complicated.